Lithography system bandwidth control

ABSTRACT

Methods and apparatus for controlling laser firing timing and hence bandwidth in a laser capable of operating at any one of multiple repetition rates.

FIELD

The present disclosed subject matter relates to control oflaser-generated light sources such as are used for integrated circuitphotolithographic manufacturing processes.

BACKGROUND

Laser radiation for semiconductor photolithography is typically suppliedas a series of pulses at a specified repetition rate, for example, inthe range of about 500 Hz to about 6 kHz. It is useful to provide a userwith the option of operating the laser at any one of the number ofrepetition rates. There are, however, engineering challenges that arisefrom providing this flexibility. One such challenge arises from the factthat lasers may exhibit a phenomenon known as bandwidth resonance thatis a function of the repetition rate (frequency) at which laser isoperated. Resonances within chamber may occur at some repetition ratesand cause sharp increases in performance metrics, (e.g. bandwidth,pointing and divergence) near the resonant frequencies, with low valleysor floors at frequencies adjacent to the resonance. The presence ofresonances per se may be tolerable provided all the data points remainwithin specification, although additional time and effort may berequired during alignment to keep the performance metrics withinspecification. In addition, peak-to-valley differences in performancemetrics caused by resonances may create technical challenges for scannerdesign and control.

One system for generating laser radiation at frequencies useful forsemiconductor photolithography (deep-ultraviolet (DUV) wavelengths)involves use of a Master Oscillator Power Amplifier (MOPA)dual-gas-discharge-chamber configuration. Bandwidth in such aconfiguration may be managed using a fast bandwidth actuator thatcontrols bandwidth by controlling the relative timing of the pulse(firing) in the master oscillator (MO) portion of the MOPA with respectto the pulse (firing) in the power amplifier (PA) portion of the MOPA.This relative timing is variously referred to as MOPA timing, AtMOPA, orDtMOPA. These shorthand notations are used herein as a shorthand for anydifferential timing control of the firing of the discharges in a seedlaser, such as the MO, and an amplifier laser, such as the PA or PO, orother amplifier laser configuration, and are not limited to a specificconfiguration such as a MOPA configuration. Such a fast bandwidthactuator is disclosed, for example, in U.S. Pat. No. 7,822,084, issuedOct. 26, 2010 and titled “Method and Apparatus for Stabilizing andTuning the Bandwidth of Laser Light”, the entire specification of whichis hereby incorporated by reference. Other examples can be found in U.S.Pat. No. 8,144,739, titled “System Method and Apparatus for Selectingand Controlling Light Source Bandwidth,” and issued on Mar. 27, 2012,the entire specification of which is herein incorporated by reference.The fast bandwidth actuator is intended to control for any bandwidthdisturbances including those occurring due to bandwidth resonance.However this fast bandwidth actuator is typically implemented as afeedback controller and so responds only after there has been an actualdisturbance. Switching from one repetition rate to another, however, mayintroduce an instantaneous disturbance in the bandwidth which ispotentially large enough to drive the bandwidth metrics out ofspecification. There is thus a need to mitigate the risk of largebandwidth transients due to repetition rate changes.

As noted, it is possible to provide bandwidth control by using MOPAtiming as the fast actuator. This actuator has a limited range and socan handle only a limited amount of disturbance in bandwidth. Due tovarious reasons, including operating repetition rate, the steady statebandwidth may change over time, for example, as a function of the amountof time since a laser chamber was last replenished with gas (refill).This change adds a constant offset in bandwidth which is thencompensated by the fast bandwidth actuator. The offset can be largeenough that it causes the MOPA timing to saturate at one end of itsrange leaving little or no room for handling normal variation inbandwidth. This problem is ameliorated by an active spectral control(ASC) desaturation technology that utilizes active bandwidthstabilization (ABS) which slowly introduces bandwidth offset until theMOPA timing actuator is near the center of its active range. Activespectral control is disclosed, for example, in U.S. Pat. No. 8,098,698,issued Jan. 17, 2012 and titled “Active Spectral Control of DUV LaserLight Source”, the entire specification of which is hereby incorporatedby reference. The solution is also affected when the laser is configuredto be able to run at any one of various repetition rates, that is, whenthe laser is repetition-rate agile. Different repetition rates mayexhibit different respective offsets in bandwidth. Centering the MOPAtiming for one repetition rate may make other repetition ratesoff-centered in a way that can result in potentially increased bandwidtherrors when switching between extremely different repetition rates.

It is desirable to obviate or mitigate at least one of the problems,whether identified herein or elsewhere, or to provide an alternative toexisting apparatus or methods.

SUMMARY

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of the present invention. Thissummary is not an extensive overview of all contemplated embodiments,and is not intended to identify key or critical elements of allembodiments nor delineate the scope of any or all embodiments. Its solepurpose is to present some concepts of one or more embodiments in asimplified form as a prelude to the more detailed description that ispresented later.

According to one aspect, transients in bandwidth due to changes inrepetition rate are reduced or prevented by using a feedforward model ofDtMOPA vs. repetition rate where DtMOPA is the steady state value of thefast bandwidth actuator at a certain repetition rate operated for aperiod of time that is sufficiently long to allow transients to settledown. This feedforward model can be initialized or calibrated byperforming a repetition rate scan, and obtaining and storing the settledvalues for DtMOPA for each repetition rate.

According to another aspect, a calibration is performed after refill inwhich a repetition rate scan is performed, building a lookup table ofMOPA timing vs. repetition rate while bandwidth is locked. The scannedrepetition rates are partitioned into bins (e.g., about 10 Hz) and oneMOPA timing value is assigned to each repetition rate bin. It is ensuredthat the MOPA timing value stored in the lookup table is not at thecontrollable limits to preserve bandwidth control margin. Whendesaturation is active, the active bandwidth stabilization is controlledin response to a difference between actual MOPA timing at a repetitionrate and the value from the lookup table for that repetition rate. Ifthe resonance behavior remains unchanged then desaturation will performseamlessly across repetition rate transitions. If the resonance behaviorchanges then the bandwidth offset introduced by the active bandwidthstabilization is adjusted.

According to another aspect, desaturation is performed without any apriori information about repetition rate dependent resonance behavior.Instead, desaturation is performed at least partially on the basis of auser's firing pattern, i.e., pattern of selection of repetition rateswhich may involve switching across several repetition rates. This willresult in MOPA timing jumping among various steady state values, onecorresponding to each repetition rate. If one of the repetition rates issuch that MOPA timing is close to the limit of its controllable rangethen active bandwidth stabilization is controlled to offset MOPA timingaway from that limit. This reestablishes the margin for the MOPA timingrange.

According to another aspect, disclosed is an apparatus comprising alaser configured to operate at any one of a plurality of repetitionrates, a bandwidth controller configured to generate a control signal toat least partially control a bandwidth of the laser, a correlatorcomprising electronically stored feedforward correlation datacorrelating a value of the control signal to a repetition rate for eachof the plurality of repetition rates, and a module configured todetermine at least one operating parameter of the laser and forsupplying the determined operating parameter to the correlator as afeedforward value, wherein the correlator is configured to generate anadjustment to the control signal based at least in part on the storedfeedforward value. The laser may have a first chamber and a secondchamber in which case the control signal may be a firing timing controlsignal DtMOPA that at least partially controls a timing of firing in thesecond chamber relative to a timing of firing in the first chamber. Thecorrelator may be configured to generate an adjustment to DtMOPA. Theadjustment to DtMOPA may be according to the formulaDtMOPA+feedforward gain*(FF(RRcurrent)−FF(RRprevious))

where

DtMOPA is the latest actual relative timing of firing in the first andsecond chamber,

FF(RRcurrent) is the stored value of DtMOPA for the current repetitionrate,

FF(RRprevious) is the stored value of DtMOPA for the previous repetitionrate, and

feedforward gain is a gain factor.

According to another aspect, the correlator may be a feedforward lookuptable storing correlation data correlating a value of the control signalto a repetition rate for each of the plurality of repetition rates. Theat least one operating parameter is an average value of DtMOPA for thecurrent repetition rate.

According to another aspect, disclosed is a method comprising the stepsof determining a current repetition rate at which a laser is firing,determining if the current repetition rate is substantially the same asan immediately prior repetition rate, and altering an operatingparameter of the laser if it is determined that the current repetitionrate is not substantially the same as the immediately prior repetitionrate. The laser may have a first chamber and a second chamber in whichcase the operating parameter may be a firing timing control value DtMOPAthat at least partially controls a timing of firing in the secondchamber in relation to a timing of firing in the first chamber. Themethod may comprise an additional step of determining an amount ofelapsed time between the last time the repetition rate used wassubstantially the same as the current repetition rate in which case thealtering step may comprise altering the operating parameter at leastpartially on the basis of the step of determining an amount of elapsedtime. The step of altering the operating parameter at least partially onthe basis of determining an amount of elapsed time comprises altering afeedforward gain.

According to another aspect, disclosed is a method comprising the stepsof detecting a change in a repetition rate for a laser capable ofoperating at any one of a plurality of repetition rates, computing afirst operating parameter of the laser since the detected change inrepetition rate, and updating, at least partially based on the computedfirst operating parameter, electronically stored correlation datacorrelating a value of a second operating parameter to a repetition ratefor each of the plurality of repetition rates. The laser may have afirst chamber and a second chamber in which case the second operatingparameter may be a timing parameter DtMOPA relating to the time offiring in the second chamber in relation to a timing of firing in thefirst chamber. The electronically stored correlation data may comprise afeedforward lookup table storing correlation data correlating a value ofDtMOPA to a repetition rate for each of the plurality of repetitionrates. The operating parameter may be an average value of DtMOPA or abandwidth error. The updating step may be carried out using therelationshipsFF[rrbin]=FF[rrbin]+gain*(ΔDTMopaavg−ΔFF)ΔDTMopaavg=DtMOPAavg(current RR)−DtMOPAavg(previous RR))ΔFF=FF[RR]−FF[RR last]

-   -   where    -   FF[rrbin] is a value stored for relative timing of firing in the        bin associated with the repetition rate rr,

DtMOPAavg(current RR) is the average timing value for the currentrepetition rate,

DtMOPAavg(previous RR) is the average timing value for an immediatelyprior repetition rate,

FF[RR] is a value stored for relative timing of firing in the binassociated with the repetition rate RR, and

FF[RR last] is a value stored for relative timing of firing in the binassociated with an immediately prior repetition rate RRlast.

The above averages may be calculated as moving averages taken over awindow of a number of pulses. The number of pulses is preferably smallbut sufficient to provide a representative average.

According to another aspect, disclosed is a method comprising the stepsof detecting a change in a repetition rate from a first repetition rateto a second repetition rate for a laser capable of operating at any oneof a plurality of repetition rates, determining when a change inrepetition rate is detected whether a control parameter at the secondrepetition rate is sufficiently different from a saturation value of thecontrol parameter, modifying the control parameter if it is determinedthat the control parameter at the second repetition rate is notsufficiently different from a saturation value of the control parameterby adjusting an operating parameter to obtain an adjusted operatingparameter such that the control parameter is sufficiently different fromthe saturation value. The laser may have a first chamber and a secondchamber in which case the control parameter may be a timing parameterDtMOPA relating to the time of firing in the second chamber in relationto a timing of firing in the first chamber. The operating parameter maybe the bandwidth offset introduced by the ABS technology.

According to another aspect, disclosed is a method comprising the stepsof detecting a change in a repetition rate from a first repetition rateto a second repetition rate for a laser capable of operating at any oneof a plurality of repetition rates and if a change in repetition rate isdetected then adjusting an operating parameter such that a controlparameter is adjusted to a reference value for the second repetitionrate. The laser may have a first chamber and a second chamber in whichcase the control parameter may be a timing parameter DtMOPA relating tothe time of firing in the second chamber in relation to a timing offiring in the first chamber. The operating parameter may be thebandwidth offset introduced by the ABS technology.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings. It is noted that the present invention is not limited to thespecific embodiments described herein. Such embodiments are presentedherein for illustrative purposes only. Additional embodiments will beapparent to persons skilled in the relevant art(s) based on theteachings contained herein.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of thepresent invention and to enable a person skilled in the relevant art(s)to make and use the present invention.

FIG. 1 shows a schematic, not to scale, view of an overall broadconception of a photolithography system according to an aspect of thedisclosed subject matter.

FIG. 2 shows a schematic, not to scale, view of an overall broadconception of an illumination system according to an aspect of thedisclosed subject matter.

FIG. 3 shows a schematic, not to scale, view of an overall broadconception of system for controlling DtMOPA as a function of repetitionrate and bandwidth error.

FIGS. 4A and 4B are flowcharts depicting a method of controlling DtMOPAas a function of repetition rate according to an aspect of the disclosedsubject matter.

FIG. 5A is a graphical representation of an overall broad conception ofa method of controlling a range of adjustment of DtMOPA according to anaspect of the disclosed subject matter and FIG. 5B is a flowchartdepicting a method of controlling DtMOPA as a function of repetitionrate according to an aspect of the disclosed subject matter.

FIG. 6A is a graphical representation of an overall broad conception ofa method of controlling a range of adjustment of DtMOPA according toanother aspect of the disclosed subject matter and FIG. 6B is aflowchart depicting a method of controlling DtMOPA as a function ofrepetition rate according to an aspect of the disclosed subject matter.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

Various embodiments are now described with reference to the drawings,wherein like reference numerals are used to refer to like elementsthroughout. In the following description, for purposes of explanation,numerous specific details are set forth in order to promote a thoroughunderstanding of one or more embodiments. It may be evident in some orall instances, however, that any embodiment described below can bepracticed without adopting the specific design details described below.In other instances, well-known structures and devices are shown in blockdiagram form in order to facilitate description of one or moreembodiments. The following presents a simplified summary of one or moreembodiments in order to provide a basic understanding of theembodiments. This summary is not an extensive overview of allcontemplated embodiments, and is not intended to identify key orcritical elements of all embodiments nor delineate the scope of any orall embodiments.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the present invention may be implemented in hardware,firmware, software, or any combination thereof. Embodiments of thepresent invention may also be implemented as instructions stored on amachine-readable medium, which may be read and executed by one or moreprocessors. A machine-readable medium may include any mechanism forstoring or transmitting information in a form readable by a machine(e.g., a computing device). For example, a machine-readable medium mayinclude read only memory (ROM); random access memory (RAM); magneticdisk storage media; optical storage media; flash memory devices;electrical, optical, acoustical or other forms of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers. Further, firmware, software, routines, instructions may bedescribed herein as performing certain actions. However, it should beappreciated that such descriptions are merely for convenience and thatsuch actions in fact result from computing devices, processors,controllers, or other devices executing the firmware, software,routines, instructions, etc.

Before describing such embodiments in more detail, it is instructive topresent an example environment in which embodiments of the presentinvention may be implemented.

Referring to FIG. 1, a photolithography system 100 that includes anillumination system 105. As described more fully below, the illuminationsystem 105 includes a light source that produces a pulsed light beam 110and directs it to a photolithography exposure apparatus or scanner 115that patterns microelectronic features on a wafer 120. The wafer 120 isplaced on a wafer table 125 constructed to hold wafer 120 and connectedto a positioner configured to accurately position the wafer 120 inaccordance with certain parameters.

The photolithography system 100 uses a light beam 110 having awavelength in the deep ultraviolet (DUV) range, for example, withwavelengths of 248 nanometers (nm) or 193 nm. The size of themicroelectronic features patterned on the wafer 120 depends on thewavelength of the light beam 110, with a lower wavelength resulting in asmaller minimum feature size. When the wavelength of the light beam 110is 248 nm or 193 nm, the minimum size of the microelectronic featurescan be, for example, 50 nm or less. The bandwidth of the light beam 110can be the actual, instantaneous bandwidth of its optical spectrum (oremission spectrum), which contains information on how the optical energyof the light beam 110 is distributed over different wavelengths. Thescanner 115 includes an optical arrangement having, for example, one ormore condenser lenses, a mask, and an objective arrangement. The mask ismovable along one or more directions, such as along an optical axis ofthe light beam 110 or in a plane that is perpendicular to the opticalaxis. The objective arrangement includes a projection lens and enablesthe image transfer to occur from the mask to the photoresist on thewafer 120. The illumination system 105 adjusts the range of angles forthe light beam 110 impinging on the mask. The illumination system 105also homogenizes (makes uniform) the intensity distribution of the lightbeam 110 across the mask.

The scanner 115 can include, among other features, a lithographycontroller 130, air conditioning devices, and power supplies for thevarious electrical components. The lithography controller 130 controlshow layers are printed on the wafer 120. The lithography controller 130includes a memory that stores information such as process recipes. Aprocess program or recipe determines the length of the exposure on thewafer 120, the mask used, as well as other factors that affect theexposure. During lithography, a plurality of pulses of the light beam110 illuminates the same area of the wafer 120 to constitute anillumination dose.

The photolithography system 100 also preferably includes a controlsystem 135. In general, the control system 135 includes one or more ofdigital electronic circuitry, computer hardware, firmware, and software.The control system 135 also includes memory which can be read-onlymemory and/or random access memory. Storage devices suitable fortangibly embodying computer program instructions and data include allforms of non-volatile memory, including, by way of example,semiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices; magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM disks.

The control system 135 can also include one or more input devices (suchas a keyboard, touch screen, microphone, mouse, hand-held input device,etc.) and one or more output devices (such as a speaker or a monitor).The control system 135 also includes one or more programmableprocessors, and one or more computer program products tangibly embodiedin a machine-readable storage device for execution by one or moreprogrammable processors. The one or more programmable processors caneach execute a program of instructions to perform desired functions byoperating on input data and generating appropriate output. Generally,the processors receive instructions and data from the memory. Any of theforegoing may be supplemented by, or incorporated in, specially designedASICs (application-specific integrated circuits). The control system 135can be centralized or be partially or wholly distributed throughout thephotolithography system 100.

Referring to FIG. 2, an exemplary illumination system 105 is a pulsedlaser source that produces a pulsed laser beam as the light beam 110.FIG. 2 depicts one particular assemblage of components and optical pathstrictly for purposes of facilitating the description of the broadprinciples of the invention in general, and it will be apparent to onehaving ordinary skill in the art that the principles of the inventionmay be advantageously applied to lasers having other components andconfigurations.

FIG. 2 shows illustratively and in block diagram a gas discharge lasersystem according to an embodiment of certain aspects of the disclosedsubject matter. The gas discharge laser system may include, e.g., asolid state or gas discharge seed laser system 140, a poweramplification (“PA”) stage, e.g., a power ring amplifier (“PRA”) stage145, relay optics 150 and laser system output subsystem 160. The seedsystem 140 may include, e.g., a master oscillator (“MO”) chamber 165, inwhich, e.g., electrical discharges between electrodes (not shown) maycause lasing gas discharges in a lasing gas to create an invertedpopulation of high energy molecules, e.g., including Ar, Kr, or Xe toproduce relatively broad band radiation that may be line narrowed to arelatively very narrow bandwidth and center wavelength selected in aline narrowing module (“LNM”) 170, as is known in the art.

The seed laser system 140 may also include a master oscillator outputcoupler (“MO OC”) 175, which may comprise a partially reflective mirror,forming with a reflective grating (not shown) in the LNM 170, anoscillator cavity in which the seed laser 140 oscillates to form theseed laser output pulse, i.e., forming a master oscillator (“MO”). Thesystem may also include a line-center analysis module (“LAM”) 180. TheLAM 180 may include an etalon spectrometer for fine wavelengthmeasurement and a coarser resolution grating spectrometer. A MOwavefront engineering box (“WEB”) 185 may serve to redirect the outputof the MO seed laser system 140 toward the amplification stage 145, andmay include, e.g., beam expansion with, e.g., a multi prism beamexpander (not shown) and coherence busting, e.g., in the form of anoptical delay path (not shown).

The amplification stage 145 may include, e.g., a lasing chamber 200,which may also be an oscillator, e.g., formed by seed beam injection andoutput coupling optics (not shown) that may be incorporated into a PRAWEB 210 and may be redirected back through the gain medium in thechamber 200 by a beam reverser 220. The PRA WEB 210 may incorporate apartially reflective input/output coupler (not shown) and a maximallyreflective mirror for the nominal operating wavelength (e.g., at around193 nm for an ArF system) and one or more prisms.

A bandwidth analysis module (“BAM”) 230 at the output of theamplification stage 145 may receive the output laser light beam ofpulses from the amplification stage and pick off a portion of the lightbeam for metrology purposes, e.g., to measure the output bandwidth andpulse energy. The laser output light beam of pulses then passes throughan optical pulse stretcher (“OPuS”) 240 and an output combinedautoshutter metrology module (“CASMM”) 250, which may also be thelocation of a pulse energy meter. One purpose of the OPuS 240 may be,e.g., to convert a single output laser pulse into a pulse train.Secondary pulses created from the original single output pulse may bedelayed with respect to each other. By distributing the original laserpulse energy into a train of secondary pulses, the effective pulselength of the laser can be expanded and at the same time the peak pulseintensity reduced. The OPuS 240 can thus receive the laser beam from thePRA WEB 210 via the BAM 230 and direct the output of the OPuS 240 to theCASMM 250.

One way that bandwidth is controlled in a system such as that justdescribed is by controlling the relative firing time, DtMOPA, of the twolaser chambers, i.e., the seed stage master oscillator MO chamber withrespect to the power amplifier PA chamber. This is shown schematicallyin FIG. 3. A controller 300 specifies a value of DtMOPA to be used by alaser 310. In the system shown, the signal from the controller 300 isadjusted at a summing junction 320 by a value ΔDTM from a feedforwardtable 330. ΔDTM represents the change in DtMOPA value stored in thefeedforward table for (1) the then current repetition rate and (2) therepetition rate prior to the current repetition rate. In a purefeedforward model, the value DtMOPA would be based on an a prioriunderstanding of the resonance behavior of the laser 310. During regularlaser operation, the repetition rate can be identified on the secondpulse of a burst. If the repetition rate has changed from theimmediately previous burst then the feedforward model is used to computethe difference between DtMOPA for the new repetition rate and DtMOPA forthe previous repetition rate. This difference is then applied to thecurrent value of DtMOPA. The result will be an instantaneous change inDtMOPA that will help prevent large transients in bandwidth. An exampleof part of a structure for a feedforward table may be as follows:

Bin number (n) Repetition rate (rr) (kHz) DtMOPA 1 3.990 DtMOPA₁ 2 4.000DtMOPA₂ 3 4.010 DtMOPA₃ 4 4.020 DtMOPA₄

The effectiveness of feedforward control depends to the extent to whichthe feedforward model is a good representation of the inherent resonancebehavior of the laser. As a practical matter, the inherent resonancebehavior may change, and it is possible that a feedforward model will bebased on an assumed inherent resonance behavior that is no longer valid.This can produce spurious effects and potentially make the transientsworse. To mitigate this risk, according to an embodiment there isprovided a method to learn new resonance behavior. This method involvesfeeding back average DtMOPA after feedforward has been applied. Theadaptive mechanism can use this average DtMOPA to update the feedforwardtable to reduce the magnitude of transient errors the next time the samerepetition rate is visited from a different one.

According to another embodiment, a feedforward gain is used thatcontrols how much of the previously learned resonance behavior is usedfor feedforward. As an example, the feedforward gain can be a multiplierwith a value between 0 and 1, with 0 meaning no feedforward is used.This feedforward gain can depend, for example, on the amount of timethat has elapsed since a most recent previous use of (visit to) the samerepetition rate. A feedforward adaptation that has been used recentlymay be assumed to still be valid, so it may be assigned a higher gain.Conversely a feedforward adaptation that has not been used recently maybe less likely to still be valid, so it may be assigned a lower gain. Inother words, the feedforward gain may decrease monotonically as theduration of a time interval between visitation to the same repetitionrate increases. Conversely, the feedforward gain may increasemonotonically as a duration of a time interval between visitation to thesame repetition rate decreases. The feedforward gain may be some otherfunction of time, such a step function having a value of one for timeintervals below a predetermined threshold and zero for all longer timeintervals.

In the arrangement of FIG. 3, a feedforward adaptation module 340computes an average DtMOPA and supplies signals indicative of themagnitude of average DTMOPA to a feedforward table 330 as a value to beused for feedforward adaptation. These averages may be moving averagesover a window of a small number of values. The feedforward table usesthe signals to adjust the value of DtMOPA associated with repetitionrate and the value of ΔDTM supplied to the summing junction 320. Asshown, the feedforward adaptation module 340 may also be configured todetect a bandwidth error for the laser 310. Because of the provision forfeedforward it can be assumed that in most instances the bandwidth errorwill be relatively small and within an acceptable range. Bandwidth errormay be detected, however, and used for additional feedforwardadaptation. This method involves feeding back the residual error inbandwidth after feedforward has been applied. If the feedforward isinaccurate, there will be a measurable residual error in the bandwidthtransient magnitude. The adaptive mechanism can use this error to updatethe feedforward table to reduce the magnitude of transient errors thenext time the same repetition rate is visited from a different one

An example of a process to be implemented in an arrangement such as thatshown in FIG. 3 is shown in FIGS. 4A and 4B. In a step S40 the laser iscaused to fire one pulse. In a step S42 it is determined whether thepulse fired was the second pulse of a burst. Because determining arepetition rate requires at least two pulses, if the determination instep S42 is negative then the process proceeds to step S44 to determineif the then current burst of pulses has ended. If the determination instep S44 is negative then the process reverts to step S40 and the laserfires again. If the determination in step S44 is positive, then theprocess proceeds to an adaptation procedure as described in connectionwith FIG. 4B. If the determination in step S44 is if negative, then theprocess proceeds to step S46 in which the current repetition rate isdetermined.

In a step S48 it is then determined whether the repetition rate haschanged. If it is determined in step S48 that the current repetitionrate has not changed, then the process proceeds to step S50, and noadjustment is made to MOPA timing based on the process, although shouldbe understood that changes can be made to MOPA timing by otherprocedures.

If it is determined in step S48 that the repetition rate has changed,then in step S51 a feedforward gain is computed. The feedforward gain inthis example depends on the time elapsed since the same repetition ratewas visited as the current repetition rate. In one embodiment thedependency of feedforward gain on elapsed time since last visit to thesame repetition rate may be linear such that as more time elapsesbetween successive visitations to the same repetition rate, thefeedforward gain decreases and vice-versa. After computing thefeedforward gain, in step S52 DtMOPA is adjusted for the now currentrepetition rate and the current repetition rate is set to be theprevious repetition rate, i.e., the new and now current repetition rateis set to be used as the previous repetition rate for the next iterationof the procedure. One example of an adjustment to DtMOPA can beaccording to the following relationship:DtMOPA=DtMOPA+feedforward gain*(FF(RR_(current))−FF(RR_(previous)))

In words, the value of DtMOPA to be applied is equal to the most recentvalue of DtMOPA actually being used plus an adjustment based on adifference between DtMOPA from the feedforward table for the currentrepetition rate RR_(current) and the value of DtMOPA from thefeedforward table for the previous repetition rate RR_(previous), thedifference being multiplied by a feedforward gain factor.

FIG. 4B shows a process that may be used to update the feedforward tableafter the conclusion of a burst. In a step S54 the moving average ofMOPA timing and the moving average of bandwidth error (ABE) since lastrepetition rate change are computed. The average is preferably notcomputed over all the pulses since the last change in repetition rate.The average is instead preferably computed over a window of recentpulses since the last change in repetition rate. In a step S56 it isdetermined whether the repetition rate for the burst is different fromthe repetition rate prior to the last update of the feedforward table,and it is also determined whether the moving average bandwidth error wasbelow a predetermined threshold (ABE_(MIN)). If it is determined thatthe repetition rate has not changed, or that the average bandwidth errorhas exceeded the threshold, then the feedforward table is not altered instep S57. If, on the other hand, it is determined in step S56 at therepetition rate has changed, and that the average bandwidth error isless than the predetermined threshold, then the process progresses to astep S58 in which it is determined whether the feedforward table for thecurrent repetition rate has been initialized. If it is determined instep S58 that the feedforward table for the current repetition rate hasnot been initialized, then in a step S60 the feedforward table for thecurrent repetition rate is initialized with the current average DtMOPAfor the then current repetition rate. If it is determined in step S58that if the feedforward table has been initialized, then the feedforwardtable for the then current repetition rate is updated in a step S62. Onemethod for updating the feedforward table may be according to thefollowing relationship:FF[rrbin]=FF[rrbin]+gain*(ΔDTMopa_(avg)−ΔFF)whereΔDTMopa_(avg)=DtMOPA_(avg)(current RR)−DtMOPA_(avg)(previous RR))ΔFF=FF[RR]−FF[RR last]

In words, the value for the “bin” (e.g., 10 Hz bin) for the repetitionrate stored in the feedforward table may be set equal to previous binvalue for that repetition rate plus the product of (1) the gain factorand (2) a difference in average DtMOPA for current repetition rate andthe previous repetition rate less the difference of the FF[RR] and FF[RRlast], i.e., the DtMOPA values stored in the feedforward table for thecurrent and immediately previous repetition rates.

Then, in a step S64 the current repetition rate is set to be used as theprevious repetition rate for the next iteration of the procedure and theprocess reverts to step S56.

The difference between average MOPA timing before and after therepetition rate change is used because actual MOPA timing responds tonormal drift as well as resonance.

As regarding the feedback signal for adaptation and conditions foradaptation, after a change in repetition rate, if the feedforward is notaccurate there is a residual bandwidth error. It is expected that theresidual error will be compensated by normal operation of the ASC. Whennormal operation of the ASC is effective at compensation the bandwidtherror reduces to within the desired threshold (ABE_(min)). Then theconverged DtMOPA value may be considered to be desired target forfeedforward. Therefore the average DtMOPA after transient convergencecan be considered as a signal for adaptation. It is also possible to usebandwidth error as a signal, either independently of or combined withDtMOPA.

To recapitulate, according to at least one embodiment, a lookup table ofMOPA timing vs. repetition rate is created. The feedforward lookup tablesupplies a MOPA timing feedforward adjustment on a repetition ratechange. The feedforward mechanism may be configured so that it thatworks in presence of drift in DtMOPA. The feedforward lookup table maybe adapted in an automated and online fashion.

The feedforward model can also be used for repetition-rate dependentMOPA timing desaturation where the bandwidth offset can be adjustedusing the active bandwidth stabilization technology such that steadystate MOPA timing stays close to the feedforward value. This helpsprevent the desaturation logic from saturating at other repetition rateswhile desaturating at one repetition rate.

To make this procedure compatible with adaptive feedforward,desaturation could be carried out on the feedforward signal rather thanthe actual DtMOPA. Adaptive feedforward changes the reference DTMOPA foreach repetition rate. If there is a uniform drift in bandwidth acrossall the repetition rates then the adaptive feedforward will react to thedrift and effectively change the reference DTMOPA. This will be moresevere if drift is faster than the desaturation rate. In such ascenario, it is desirable to prevent the reference DTMOPA from achievingextreme values so it therefore may be desirable to provide anotherdesaturation loop on the reference DTMOPA to keep it within controllablerange while the normal desaturation loop is already active and trying tobring current DTMOPA to reference DTMOPA. The new desaturation loop onreference DTMOPA may be configured to override the regular desaturationwhenever reference DTMOPA is deemed to be too close to the extremevalues.

One form of adaptation maybe repetition rate dependent where thedesaturation target is a MOPA timing value that is different for eachrepetition rate. Another form of adaptation may be a repetition-rateindependent ASC desaturation technology where instead of there being adesaturation target the bandwidth offset is adjusted using the automaticbandwidth stabilization technology in response to MOPA timingapproaching its limits.

These processes will now be explained in connection with FIGS. 5A and 5Band FIGS. 6A and 6B. Referring first to FIG. 5A, a curve showing thedependence of DtMOPA on repetition rate are shown. In the background,the vertically middle, lightly shaded region is the range in whichDtMOPA is most easily controllable. The upper and lower more darklyshaded regions are areas of marginal DtMOPA controllability, i.e.,approaching saturation. If the user is operating at the repetition ratedesignated by “1”, then DtMOPA is comfortably within its controllablezone. If the user switches to repetition rate 2, however, then DtMOPAwill be outside of its controllable zone (saturated). In this case, theactive bandwidth stabilization adds an offset that moves thecontrollable zone so that DtMOPA v. repetition rate curve is againwithin the controllable zone as represented by the topmost curve. If theuser then switches to then switches to repetition rate 3 then DtMOPA atthis repetition rate is above the upper margin and the active bandwidthstabilization adds an offset that moves the controllable zone untilDtMOPA at this repetition rate is within the lightly shaded region. Thisresults in a relationship as shown in the middle curve with respect tothe background. This process continues for each repetition rate at whichuser operates the laser. If the controllable range is sufficient thenultimately, DtMOPA for all the visited repetition rates will be centeredwithin the most controllable lightly shaded region.

FIG. 5B is a flowchart illustrating the steps of a possibleimplementation of this process. In step S70 it is determined whetherDtMOPA is in its controllable range for the current repetition rate. IfDtMOPA is in its controllable range for the current repetition rate thenthe process requires no further action (step S72). If DtMOPA is not inits in its controllable range for the current repetition rate then instep S74 the ABS bandwidth offset is adjusted to move DtMOPA back intothe controllable range in step S74.

As an alternative, and referring first to FIG. 6A, a curve showing thedependence of DtMOPA on repetition rate is shown. In the background, thevertically middle, lightly shaded region is the range in which DtMOPA ismost easily controllable. The upper and lower more darkly shaded regionsare areas of marginal DtMOPA controllability, i.e., approachingsaturation. In this method there is a DtMOPA reference value for eachrepetition rate, as shown by the dashed line. If the user is operatingat the repetition rate designated by “1”, then DtMOPA is adjusted tomatch the reference value at that repetition rate. If the user switchesto repetition rate 2, then DtMOPA is again adjusted to match thereference value at that repetition rate. If the user then switches tothen switches to repetition rate 3 then DtMOPA then DtMOPA is againadjusted to match the reference value at that repetition rate. Thisprocess continues for each repetition rate at which user operates thelaser. If controllable range is sufficient and the bandwidth resonancebehavior with respect to repetition rate does not change in shape thenultimately, DtMOPA for all the visited repetition rates will match thereference DtMOPA.

FIG. 6B is a flowchart illustrating the steps of a possibleimplementation of this process. In step S76 a current repetition rate isobtained. In step S78 it is determined whether the actual DtMOPA forthat repetition rate is different from the reference DtMOPA for thatrepetition rate. If the actual DtMOPA for that repetition rate is thesame as the reference DtMOPA for that repetition rate then processrequires no further action (step S80). If. If the actual DtMOPA for thatrepetition rate is the different from the reference DtMOPA for thatrepetition rate then the ABS offset is adjusted to make the actualDtMOPA for that repetition rate the same as the reference DtMOPA forthat repetition rate.

The above description includes examples of multiple embodiments. It is,of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing theaforementioned embodiments, but one of ordinary skill in the art mayrecognize that many further combinations and permutations of variousembodiments are possible. Accordingly, the described embodiments areintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.Furthermore, to the extent that the term “includes” is used in eitherthe detailed description or the claims, such term is intended to beinclusive in a manner similar to the term “comprising” as “comprising”is construed when employed as a transitional word in a claim.Furthermore, although elements of the described aspects and/orembodiments may be described or claimed in the singular, the plural iscontemplated unless limitation to the singular is explicitly stated.Additionally, all or a portion of any aspect and/or embodiment may beutilized with all or a portion of any other aspect and/or embodiment,unless stated otherwise.

The invention claimed is:
 1. Apparatus comprising: a laser configured to operate at any one of a plurality of repetition rates; a bandwidth controller configured to generate a control signal to at least partially control a bandwidth of the laser; a correlator comprising electronically stored feedforward correlation data correlating a value of the control signal to each of the plurality of repetition rates; and a module configured to determine at least one operating parameter of the laser and for supplying the determined operating parameter to the correlator as a feedforward value, wherein the correlator is configured to generate an adjustment to the control signal based at least in part on the feedforward value.
 2. Apparatus as claimed in claim 1 wherein the laser has a first chamber and a second chamber and wherein the control signal is a firing timing control signal DtMOPA that at least partially controls a timing of firing in the second chamber relative to a timing of firing in the first chamber.
 3. Apparatus as claimed in claim 2 wherein the correlator is configured to generate an adjustment to DtMOPA.
 4. Apparatus as claimed in claim 1 wherein the correlator is a feedforward lookup table storing correlation data correlating a value of the control signal to a repetition rate for at least some of the plurality of repetition rates.
 5. Apparatus comprising: a laser having a first chamber and a second chamber and configured to operate at any one of a plurality of repetition rates; a bandwidth controller configured to generate a control signal to at least partially control a bandwidth of the laser, the control signal comprising a firing timing control signal DtMOPA that at least partially controls a timing of firing in the second chamber relative to a timing of firing in the first chamber; a correlator comprising electronically stored feedforward correlation data correlating a value of the control signal to a repetition rate for each of the plurality of repetition rates; and a module configured to determine at least one operating parameter of the laser and for supplying the determined operating parameter to the correlator as a feedforward value, wherein the correlator is configured to generate an adjustment to the control signal based at least in part on the feedforward value the correlator is configured to generate an adjustment to DtMOPA according to the formula DtMOPA+feedforward gain*(FF(RRcurrent)−FF(RRprevious)) where DtMOPA is an actual relative timing of firing in the first and second chamber, FF(RRcurrent) is a stored value of DtMOPA for a current repetition rate, FF(RRprevious) is a stored value of DtMOPA for a previous repetition rate, and feedforward gain is a gain factor.
 6. Apparatus comprising: a laser having a first chamber and a second chamber and configured to operate at any one of a plurality of repetition rates; a bandwidth controller configured to generate a control signal to at least partially control a bandwidth of the laser, the control signal comprising a firing timing control signal DtMOPA that at least partially controls a timing of firing in the second chamber relative to a timing of firing in the first chamber; a correlator comprising electronically stored feedforward correlation data correlating a value of the control signal to a repetition rate for each of the plurality of repetition rates; and a module configured to determine at least one operating parameter of the laser and for supplying the determined operating parameter to the correlator as a feedforward value, wherein the at least one operating parameter is a moving average value of DtMOPA for the current repetition rate.
 7. Apparatus comprising: a laser configured to operate at any one of a plurality of repetition rates; a bandwidth controller configured to generate a control signal to at least partially control a bandwidth of the laser; a module configured to determine at least one operating parameter of the laser and for generating a signal indicative of the determined operating parameter as a feedforward value; and a correlator arranged to receive a signal indicative of a current repetition rate and the feedforward value and comprising electronically stored feedforward correlation data correlating a value of the control signal to each of the plurality of repetition rates, wherein the correlator is configured to generate an adjustment to the control signal based at least in part on the feedforward value.
 8. Apparatus as claimed in claim 7 wherein the laser has a first chamber and a second chamber and wherein the control signal is a firing timing control signal DtMOPA that at least partially controls a timing of firing in the second chamber relative to a timing of firing in the first chamber.
 9. Apparatus as claimed in claim 8 wherein the correlator is configured to generate an adjustment to DtMOPA.
 10. Apparatus as claimed in claim 7 wherein the correlator is a feedforward lookup table storing correlation data correlating a value of the control signal to a repetition rate for at least some of the plurality of repetition rates. 